1. Field of the Invention
The present invention relates to a transmission of data in a discrete multi-tone (DMT) data communications network, particularly a transmission and reception of data in a network for communications between multiple devices coupled to existing wiring, for example twisted pair telephone wiring in the user""s residence.
2. Description of the Related Art
Modem society continues to create exponentially increasing demands for digital information and the communication of such information between data devices. Local area networks use a network, cable or other media to link stations on the network for exchange of information in the form of packets of digital data. A typical local area network architecture uses a media access control (MAC) enabling network interface cards at each station to share access to the media. Conventional local area network architectures use media access controllers operating according to half-duplex or full-duplex Ethernet (ANSI/IEEE standard 802.3) protocol using a prescribed network medium, such as 10 BASE-T.
These architectures have proven quite successful in providing data communications in commercial applications. However, these common local area network architectures require installation of specialized wiring and use of specific wiring topologies. For example, the most popular network protocols, such as Ethernet, require special rules for the wiring, for example with regard to quality of wire, range of transmission and termination.
Due to the success of the Internet and the rapid decreases in the prices of personal computers and associated data equipment, a demand has arisen for data communications between a limited number of devices within relatively small premises, typically a residence or small business. While existing local area networks can serve the purpose, in such installations, the cost of installing physical network wiring satisfying the rules for the particular protocol can be prohibitively expensive.
Most existing buildings, including residences, include some existing wiring, for phones, electrical power and the like. Proposals have been made to communicate data using such existing infrastructure. This reduces the costs of wiring for the network, but the existing wiring raises a variety of issues regarding transport of high-speed digital signals.
For example, efforts are underway to develop an architecture that enables computers to be linked together using conventional twisted pair telephone lines. Such an arrangement, referred to herein as a home network environment, provides the advantage that existing telephone wiring in a home may be used to implement a home network environment without incurring costs for substantial new wiring installation. However, any such network must deal with issues relating to the specific nature of in-home telephone wiring, such as operation over a media shared with other services without interference from or interfering with the other services, irregular topology, and noise. With respect to the noise issue, every device on the telephone line may be a thermal noise source, and the wiring may act much like an antenna to pick up disruptive radio signal noise. Telephone lines are inherently noisy due to spurious noise caused by electrical devices in the home, for example dimmer switches, transformers of home appliances, etc. In addition, the twisted pair telephone lines suffer from turn-on transients due to on-hook and off-hook and noise pulses from the standard telephones coupled to the lines, and electrical systems such as heating and air conditioning systems, etc.
An additional problem in telephone wiring networks is that the signal condition (i.e., shape) of a transmitted waveform depends largely on the wiring topology. Numerous branch connections in the twisted pair telephone line medium, as well as the different associated lengths of the branch connections, may cause multiple signal reflections on a transmitted network signal. Telephone wiring topology may cause the network signal from one network station to have a peak-to-peak voltage on the order of 10 to 20 millivolts, whereas network signals from another network station may have a value on the order of one to two volts. Hence, the amplitude and shape of a received pulse may be so distorted that recovery of a transmit clock or transmit data from the received pulse becomes substantially difficult.
At the same time a number of XDSL technologies are being developed and are in early stages of deployment, for providing substantially higher rates of data communication over twisted pair telephone wiring of the telephone network. XDSL here is used as a generic term for a group of higher-rate digital subscriber line communication schemes capable Of utilizing twisted pair wiring from an office or other terminal node of a telephone network to the subscriber premises. Examples under various stages of development include ADSL (Asymmetrical Digital Subscriber Line), HDSL (High data rate Digital Subscriber Line) and VDSL (Very high data rate Digital Subscriber Line).
Consider ADSL as a representative example. For an ADSL related service, the user""s telephone network carrier installs one ADSL modem unit at the network end of the user""s existing twisted-pair copper telephone wiring. Typically, this modem is installed in the serving central office or in the remote terminal of a digital loop carrier system. The user obtains a compatible ADSL modem and connects that modem to the customer premises end of the telephone wiring. The user""s computer connects to the modem. The central office modem is sometimes referred to as an ADSL Terminal Unitxe2x80x94Central Office or xe2x80x98ATU-Cxe2x80x99. The customer premises modem is sometimes referred to as an ADSL Terminal Unitxe2x80x94Remote or xe2x80x98ATU-Rxe2x80x99. The ADSL user""s normal telephone equipment also connects to the line, either directly or through a frequency combiner/splitter, which often is incorporated in the ATU-R. The normal telephone signals are split off at both ends of the line and processed in the normal manner.
For digital data communication purposes, the ATU-C and ATU-R modem units create at least two logical channels in the frequency spectrum above that used for the normal telephone traffic. One of these channels is a medium speed duplex channel; the other is a high-speed downstream only channel. Two techniques are under development for dividing the usable bandwidth of the telephone line to provide these channels. One approach uses Echo Cancellation. Currently, the most common approach is to divide the usable bandwidth of a twisted wire pair telephone line by frequency, that is to say by Frequency Division Multiplexing (FDM).
FDM uses one frequency band for upstream data and another frequency band for downstream data. The downstream path is then divided by time division multiplexing into one or more high-speed channels and one or more low speed channels. The upstream path also may be time-division multiplexed into corresponding low speed channels.
The most common form of the FDM data transport for DSL services utilizes discrete multi-tone (DMT) technology. A DMT signal is basically the sum of N independently QAM modulated signals, each carried over a distinct carrier frequency channel. The frequency separation of each carrier is 4.3125 kHz with a total number of 256 carriers or tones (ANSI). An asymmetrical implementation of this 256 tone-carrier DMT coding scheme might use tones 32-255 to provide a downstream channel of approximately 1 MHz analog bandwidth. In such an implementation, tones 831 are used as carriers to provide an upstream channel of approximately 100 kHz analog bandwidth. Each tone is quadrature amplitude modulated (QAM) to carry up to 15 bits of data on each cycle of the tone waveform.
The existing DSL systems provide effective high-speed data communications over twisted pair wiring between customer premises and corresponding network-side units, for example located at a central office of the telephone network. The DSL modem units overcome many of the problems involved in data communication over twisted pair wiring. However, for a number of reasons, the existing DSL units are not suitable to providing local area network type communications within a customer""s premises. For example, existing ADSL units are designed for point-to-point communication. That is to say, one ATU-R at the residence communicates with one ATU-C unit on the network end of the customer""s line. There is no way to use the units for multi-point communications. Also, the existing ADSL modems tend to be quite complex, and therefore are too expensive for in-home communications between multiple data devices of one customer.
A need therefore still exists for techniques to adapt DMT type DSL communications for use over existing in-home wiring. The adaptations should enable multi-point communications. Also, many of the problems overcome by complex methodologies in ADSL communications need corresponding simpler, more cost effective solutions for in-home networking.
For example, decoding of DMT data signals requires accurate timing between the transmitter and the receiver. In existing ADSL communications, one of the tone frequency channels is used as a pilot tone channel. DMT demodulation and decoding for all other channels is based on recovery of timing information from the pilot tone. For example, FIG. 1 illustrates a conventional arrangement for modulating a bit-pair to be transmitted on a given tone As shown in FIG. 1, the bit pair is modulated (i.e., mapped) into a complex number, referred to as a constellation point; hence, the bit-pair 00 is mapped to constellation point 1+j, 01 is mapped to xe2x88x921+j, 11 is mapped to xe2x88x921xe2x88x92j, and 10 is mapped to 1xe2x88x92j, where j is the square root of xe2x88x921. The constellation point represents the amplitude and phase of the corresponding QAM-modulated tone. As shown in FIG. 1, the shaded point 5a represents the bit pair 00 transmitted onto a network mediums whereas the shaded point 5b represents the bit pair 00 having encountered attenuation and phase distortion 6 due to transmission 7 on the network medium. Hence, the receiver must perform complex equalization 9 based on the pilot tone channel to generate an equalized constellation point 5c that corresponds to the the original constellation point 5a. 
Existing systems suffer the disadvantage of requiring a pilot tone channel dedicated to providing complex attenuation information for receiver equalizers in networked systems. The necessity of a pilot tone wastes at least one tone that could otherwise be used for transmission of data. Also, coordination of reception of all of the other tones to the timing from the one pilot tone is extremely complex. Moreover, the necessity of an equalizer in the receiver system results in a high-cost and high-complexity receiver system, especially since the equalizer may need to retune itself to different coefficient settings for communication with different network nodes.
In a multi-point, random access communication application, the equalization problem becomes particularly acute. Unlike the point-to-point implementations where communications are always on-going and enable virtually continuous synchronization between transmitter and receiver, the random access type devices only send when they have data to send. As a result, the receiver needs to either be able to identify the transmitting node and quickly adjust its equalizer coefficient settings accordingly on a per-packet basis, or the receiver must use multiple equalizers, each tuned for reception of data from a corresponding network node, to simultaneously equalize the received signal and then determine the optimally-tuned signal. In addition, use of a long training sequence attached on each packet in a packet-switched network (e.g., Ethernet) is impractical due to the added overhead. Also, a transmitter clock frequency my be slightly different than a receiver clock frequency; hence, the transmitted constellation points may encounter phase rotation at the-receiver end, requiring either synchronization between the transmitter and receiver, or frequent update of the equalizer to compensate for the rotation.
Hence, the equalization problem in a multi-point, random access communication application results in a substantially complex receiver system having sophisticated (and hence expensive) equalizers.
A need therefore exists for a simpler form of transmitting and recovering data, particularly one that is readily adaptable to a multi-point network using existing wiring such as twisted pair telephone wiring on a user""s premises.
The present invention overcomes the noted problems involved in data networking and satisfies the above stated needs by providing a coding technique, at the physical layer, for use in a multi-point DMT communication system, by generating a constellation tint for a tone based on a relative position of a preceding constellation point and a value of a group of bits. In particular, a group of bits are encoded into a constellation point based on the relative position of the preceding constellation point and the value of the group of bits. Since each constellation point output from a given transmitter to a given receiver will have the substantially the same attenuation and phase distortion over a relatively short time period (e.g., within a few milliseconds or shorter), the phase differential between the preceding constellation point and the generated constellation point will remain substantially the same from the transmitter to the receiver. Hence, a receiver can reliably determine the vale of the group of bits based on the phase difference between the preceding constellation point and the generated constellation point, regardless of the distortion conditions encountered on the network media during transmission.
Thus one aspect of the present invention relates to a method for encoding data for transmission on a shared network medium in a random-access multipoint network. The method includes transmitting on the shared network medium a tone modulated based on a corresponding first constellation point having a first position in a complex planes encoding a group of bits into a second constellation point having a second position in the complex plane based on the first position and a value of the group of bits, and modulating and transmitting the tone on the shared network medium based on the second constellation point consecutively following the first constellation point. Transmission of the tone based on the second constellation point consecutively following the first constellation point ensures that a receiver, upon demodulation of the transmitted tone, recovers the second constellation point consecutively following the first constellation point, enabling the receiver to recover the group of bits based on the difference in the first and second positions of the respective constellation points, regardless of any phase distortion that may be encountered on the channel medium.
Another aspect of the present invention provides a method of communicating data over in-house wiring. The method includes transmitting from a transmitter onto the in-house wiring a first symbol as a plurality of discrete multiple tones, one of said tones modulated according to a first constellation point having a first position in a complex plane, detecting in a receiver the first constellation point at a first distorted position that is different then the first position in the complex plane, encoding in the transmitter a group of bits into a second constellation point having a second position in the complex plane based on the first position and value of the group of bits, transmitting a second symbol onto the in-house wiring by modulating the one tone according to the second constellation point consecutively following the first constellation point, detecting in the receiver a second distorted position of the second constellation point in the complex plane, and recovering the group of bits in the receiver by comparing the second distorted position of the second constellation point with the first distorted position.
Still another aspect of the present invention provides a A discrete multi-tone transmitter for transmitting digital data on an analog line. The transmitter includes a differential encoder for encoding the digital data into a new constellation point having a new position in a complex plane based on a value of the digital data and a consecutively preceding constellation point having a corresponding preceding position, and a converter for converting the consecutively preceding constellation point and the new constellation point into a time domain-modulated tone signal for transmission on the digital line.
Yet another aspect of the present invention provides a random access multipoint network for transmission of data. The network includes a shared network medium and a plurality of network nodes. Each network node includes a transmitter having a differential encoder for encoding the data into a new constellation point having a new position in a complex plane based on a value of the data and a consecutively preceding constellation point having a corresponding preceding position, and a receiver. The receiver is configured for detecting a first constellation point having a first position in a complex plane and a second constellation point, consecutively following the first constellation point and having a second position in the complex plane, from a modulated tone having encountered amplitude and/or phase distortion on the shared network medium. The receiver recovers the data transmitted by the modulated tone based on the second position relative to the first position.
Additional advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.